This invention relates to a digital differential signal transmission apparatus in which a transmission line for signals controlling a painting robot or the like is provided with an explosion-proof mechanism.
The standard RS422 of the EIA in the United States is an example of a standard used as a digital differential signal transmission method for transmitting a digital signal at high speed and over long distances.
According to this standard, first a single end input A to be transmitted is converted into a pair of differential signals, namely a signal B of the same phase and a signal C of the opposite phase, by a differential driver 11 (see FIG. 3) provided on the transmitting end. A device such as an AM26LS31 is used as the differential driver 11. The pair of differential signals is sent to a differential receiver 15, which comprises an AM26LS32 on the receiving end, by a transmission line 12, and is thereby converted into a single end output D and transmitted.
In this case, the transmission line 12 employs a line having a characteristic impedance of 100 ohms. Therefore, a filter circuit 14 comprising a resistor 13 of 100 ohms for impedance matching, a resistor 14a for improving the noise margin, and a capacitor 14b is inserted in the transmission line 12.
In an apparatus such as a painting robot used in a atmosphere containing gas that poses the danger of explosion, an explosion-proof structure is required in order to prevent the explosive gas from being ignited by a spark produced in an electric circuit.
Though there are a variety of such explosion-proof structures available, the teach pendant of a painting robot often requires what is referred to as an essential safe explosion-proof structure because of its application. An essential safe explosion-proof structure is one in which it has been verified, as by tests in public facilities, that an explosive gas will not be ignited by electric sparks produced at the time of an accident or by high-temperature parts during normal operation. At the time of abnormal operation, essential safety is assured by inserting a barrier in a signal transmission line so as to prevent the transmission of ignition energy which can lead to an explosion. The barrier referred to here is a so-called "safety device" which, as shown in FIG. 4 in the form of a shunt diode-type safety device (a Zener barrier) 16, by way of example, can be composed of voltage-limiting Zener diodes 17, a current-limiting resistor 18 and a fuse 19 for protecting the Zener diodes 17.
When a signal transmission line is inserted between a nonessential safety side in the right-hand direction of FIG. 4 and an essential safety side in the left-hand direction, energy above the limit set by the Zener barrier is not transmitted to the essential safety side and safety is assured.
FIG. 5 is a circuit diagram in which an essential safe explosion-proof structure is obtained by inserting a Zener barrier in the above-described digital differential signal transmission system.
In FIG. 5, the Zener barrier 16 described in conjunction with FIG. 4 is inserted into each of a pair of transmission lines 20 provided between the differential driver 11 and the differential receiver 15, thereby maintaining safety on the side of the differential receiver 15. However, since the transmission lines 20 have a characteristic impedance of 100 ohms, the voltage-dropping effect of the current-limiting resistor 18 of Zener barrier 16 is great and the signal level becomes small on the receiving end. A problem that results is a reduced noise margin.
Further, since the Zener barriers 16 are inserted in the transmission lines 20 at points along their lengths, mismatching develops in the characteristic impedances of the transmission lines, reflection of transmitted signals occurs and distortion is produced in the signal waveforms.
Moreover, there is a need to provide two Zener barriers in the transmission lines in one direction, as shown in FIG. 5. In two-way communications for transmitting and receiving signals, 2.times.2 Zener barriers, for a total of four, are required, as shown in FIG. 6. Use of four Zener barriers, which are comparatively high in price, raises the cost of the transmission lines.